1. Field of the Invention
The present invention is related to a mount for coupling a linear fiber array to a spectrometer slit.
2. Prior Art
A spectrometer is an optical instrument used to measure properties of light over a specific portion of the electromagnetic spectrum. The variable measured is most often the light's intensity but could also, for instance, be the polarization state. The independent variable is usually the wavelength of the light, normally expressed as some fraction of a meter, but sometimes expressed as some unit directly proportional to the photon energy, such as wavenumber or electron volts, which has a reciprocal relationship to wavelength. A spectrometer is used in spectroscopy for producing spectral lines and measuring their wavelengths and intensities.
A typical spectrometer includes an entrance slit, a collimating mirror or lens, a wavelength dispersive element, such as grating or prism, a mirror or lens for focusing the dispersed light onto a single element detector such as photomultiplier (PMT) or avalanche photodiode. The entrance and exit slits are aligned in parallel to the grating grooves or perpendicular to the dispersion direction of the prism. When a multi-element detector such as diode array or charge coupled device (CCD) is used, the exit slit is removed and the width of the diode or CCD pixel is acting as the exit slit. The slits determine the spectrometer's resolution (subject to the overriding resolving power of the dispersion medium) and have a profound effect on the amount of light that is passed through the spectrometer.
For the remote sampling applications, the signal light from sample under investigation is transmitted through a fiber optic cable with either a single large core fiber or round-to-linear multiple fiber bundle which is butt-coupled onto the entrance slit. When a round-to-linear multiple fiber bundle is used, the linear end of the fiber bundle is aligned along the entrance slit, which is achieved by rotating the ferrule and fixed by a set screw conventionally. In order to achieve better light coupling efficiency, care must be taken to align the linear fiber array to the entrance slit. This manual alignment procedure is tedious and difficult and requires skilled personnel. In addition, the mounting mechanism is not stable at all with a single set screw. Thus it limits its field use. Particularly it requires tedious realignment after each movement.
With this conventional mounting mechanism, light loss occurs at the fiber optic array and entrance slit interface. To facilitate the alignment, the fiber core diameter is generally larger than the slit width. Thus a significant portion of the light from the fiber bundle is blocked by the slit. In some cases, light loss can be as much as 90%. This greatly reduces the system throughput, sensitivity and thus signal-to-noise ratio. This presents a problem for situations like weak signal to begin with. For example, Raman scattering is extremely weak, only very small fraction of the incident light inelastically scattered while almost all of the light is scattered elastically, so called Rayleigh scattering. Therefore in Raman spectroscopy it cannot afford losing any part of the signal.
U.S. Pat. No. 5,042,893 to Ong describes a direct mount coupler for use with a diode array spectrophotometer system. In this design, the entrance slit is replaced by a slit block containing a single waveguide (or plurality of fiber optic waveguides). The waveguide has a linear cross section adjacent the spectrometer and a round cross section at the connector, which secures the signal collection fiber. By removing the slit, the linear fiber array is acting as a slit so misalignment of the fiber slit interface is eliminated. Therefore light loss at the fiber bundle and slit interface is avoided and the system provides better sensitivity. However, it introduces an additional optic component (the single waveguide or plurality of fiber optic waveguides) and an interface at the connector between the round section of the coupler and a signal fiber waveguide. There exists light loss due to fiber-fiber coupling at the connector interface. In addition, a large core fiber has to be used for transferring the signal from a sample to match the diameter of the round section at the connector according to the patent's teaching. As the core diameter of the fiber is getting larger, the fiber is getting stiffer. Therefore the care must be taken to handle it which is inconvenient to the users for routine analysis.
Therefore, there is a need for linear fiber array mount for a spectrometer to provide an efficient coupling of the fiber slit to the entrance slit. Furthermore there is a need for a fiber slit mount for a slitless spectrometer to minimize light loss during the transfer between a sample and detector.